In vertebrates thyrotropin-releasing hormone (TRH) is a highly conserved neuropeptide that exerts the hormonal control of thyroidstimulating hormone (TSH) levels as well as neuromodulatory functions. However, a functional equivalent in protostomian animals remains unknown, although TRH receptors are conserved in proto-and deuterostomians. Here we identify a TRH-like neuropeptide precursor in Caenorhabditis elegans that belongs to a bilaterian family of TRH precursors. Using CRISPR/Cas9 and RNAi reverse genetics, we show that TRH-like neuropeptides, through the activation of their receptor TRHR-1, promote growth in C. elegans. TRH-like peptides from pharyngeal motor neurons are required for normal body size, and knockdown of their receptor in pharyngeal muscle cells reduces growth. Mutants deficient for TRH signaling have no defects in pharyngeal pumping or isthmus peristalsis rates, but their growth defect depends on the bacterial diet. In addition to the decrease in growth, trh-1 mutants have a reduced number of offspring. Our study suggests that TRH is an evolutionarily ancient neuropeptide, having its origin before the divergence of protostomes and deuterostomes, and may ancestrally have been involved in the control of postembryonic growth and reproduction.thyrotropin-releasing hormone | C. elegans | neuropeptide | molecular evolution | growth regulation A fter Harris's initial proposal on the hypothalamic control of pituitary secretion (1), it took almost two decades to identify the first hypophysiotropic molecule. In 1969 the groups of Schally and Guillemin isolated the tripeptide pQHP-NH 2 (2, 3), named "thyrotropin-releasing hormone" (TRH). The sequence of TRH is fully conserved across all vertebrates, indicating that strong evolutionary pressure has acted to preserve its structure (4). In all vertebrate phyla TRH is synthesized from a larger precursor protein (preproTRH) that contains five to eight copies of the TRH sequence (4). Following the explosion of genome and transcriptome sequence data, preproTRH was identified in chordate species lacking a bona fide pituitary, e.g., cephalochordates (5), and in the genomes of more ancient deuterostomes, including echinoderms (6, 7). In contrast to vertebrate (pQHP-NH 2 ) and chordate (pQSP-NH 2 ) tripeptide TRHs, most predicted echinoderm TRHs are tetrapeptidesLike vertebrate TRH, they are small peptides with an N-terminal pyroglutamate, a C-terminal amide (-NH 2 ) group, and amino acids with aromatic or cyclic side chains at the second and third positions. TRH therefore is widely distributed throughout the deuterostomian lineage of the Animal Kingdom, suggesting an ancient origin for this neuropeptide hormone.In mammals, hypothalamic TRH is the prime regulator of the set point of thyroid-stimulating hormone (TSH) synthesis and secretion by the anterior pituitary thyrotrophs (9). TSH secretion stimulates the thyroid gland to produce the thyroid hormones (THs) thyroxine (T 4 ) and triiodothyronine (T 3 ). The hypothalamus-pituitary-thyroid (HPT) axis is essentia...

In the three decades since the FMRFamide peptide was isolated from the mollusk Macrocallista nimbosa, structurally similar peptides sharing a C-terminal RFamide motif have been identified across the animal kingdom. FMRFamide-like peptides (FLPs) represent the largest known family of neuropeptides in invertebrates. In the phylum Nematoda, at least 32 flp-genes are classified, making the FLP system of nematodes unusually complex. The diversity of the nematode FLP complement is most extensively mapped in Caenorhabditis elegans, where over 70 FLPs have been predicted. FLPs have shown to be expressed in the majority of the 302 C. elegans neurons including interneurons, sensory neurons, and motor neurons. The vast expression of FLPs is reflected in the broad functional repertoire of nematode FLP signaling, including neuroendocrine and neuromodulatory effects on locomotory activity, reproduction, feeding, and behavior. In contrast to the many identified nematode FLPs, only few peptides have been assigned a receptor and there is the need to clarify the pathway components and working mechanisms of the FLP signaling network. Here, we review the diversity, distribution, and functions of FLPs in nematodes.

Learning and memory are regulated by neuromodulatory pathways, but the contribution and temporal requirement of most neuromodulators in a learning circuit are unknown. Here we identify the evolutionarily conserved neuromedin U (NMU) neuropeptide family as a regulator of C. elegans gustatory aversive learning. The NMU homolog CAPA-1 and its receptor NMUR-1 are required for the retrieval of learned salt avoidance. Gustatory aversive learning requires the release of CAPA-1 neuropeptides from sensory ASG neurons that respond to salt stimuli in an experience-dependent manner. Optogenetic silencing of CAPA-1 neurons blocks the expression, but not the acquisition, of learned salt avoidance. CAPA-1 signals through NMUR-1 in AFD sensory neurons to modulate two navigational strategies for salt chemotaxis. Aversive conditioning thus recruits NMU signaling to modulate locomotor programs for expressing learned avoidance behavior. Because NMU signaling is conserved across bilaterian animals, our findings incite further research into its function in other learning circuits.

Aversive learning is fundamental for animals to increase chances of survival. In addition to classical neurotransmitters, neuropeptides have emerged to modulate such complex behaviors. Among them, neuropeptide Y (NPY) is well known to promote aversive memory acquisition in mammals. Here we identify an NPY/neuropeptide F (NPF)-related neuropeptide system in Caenorhabditis elegans and show that this FLP-34/NPR-11 system is required for learning negative associations, a process that is reminiscent of NPY signaling in mammals. The Caenorhabditis elegans NPY/NPF ortholog FLP-34 displays conserved structural hallmarks of bilaterian-wide NPY/NPF neuropeptides. We show that it is required for aversive olfactory learning after pairing diacetyl with the absence of food, but not for appetitive olfactory learning in response to butanone. To mediate diacetyl learning and thus integrate the aversive food context with the diacetyl odor, FLP-34 is released from serotonergic neurons and signals through its evolutionarily conserved NPY/NPF GPCR, NPR-11, in downstream AIA interneurons. NPR-11 activation in the AIA integration center results in avoidance of a previously attractive stimulus. This study opens perspectives for a deeper understanding of stress conditions in which aversive learning results in excessive avoidance.

Aversive learning and memories are crucial for animals to avoid previously encountered stressful stimuli and thereby increase their chance of survival. Neuropeptides are essential signaling molecules in the brain and are emerging as important modulators of learned behaviors, but their precise role is not well understood. Here, we show that neuropeptides of the evolutionarily conserved MyoInhibitory Peptide (MIP)-family modify salt chemotaxis behavior in Caenorhabditis elegans according to previous experience. MIP signaling, through activation of the G protein-coupled receptor SPRR-2, is required for short-term gustatory plasticity. In addition, MIP/SPRR-2 neuropeptide-receptor signaling mediates another type of aversive gustatory learning called salt avoidance learning that depends on de novo transcription, translation and the CREB transcription factor, all hallmarks of long-term memory. MIP/SPRR-2 signaling mediates salt avoidance learning in parallel with insulin signaling. These findings lay a foundation to investigate the suggested orphan MIP receptor orthologs in deuterostomians, including human GPR139 and GPR142.